Mineral Fertilizer Use and the Environment International Fertilizer Industry Association United Nations Environment Programme

14.2. Tropical and subtropical
zones
The soil content of organic matter is often
relatively low under warmer climatic conditions
due to oxidation, and the benefits of increasing
the organic matter content of soils are clearer
than under temperate conditions. Apart from its
plant nutrient content and function in improving
soil physical properties, there is evidence that
organic material can help to offset the effects of
soil acidity and aluminium toxicity, and it may
supply soil sites which hold readily available
phosphate for plant uptake. Trace element
deficiencies are increasingly common under more
intensive growing conditions and, in the absence
of more precise assessments, organic material
from outside sources may incidentally provide
some of the needs.
In trials in India, the application of manure
together with mineral fertilizers has given a clear
yield advantage. Long-term experiments showed
that after 20 years of ammonium sulphate
application, the crop yield declined to zero. NPK
plus lime sustained the highest yield and FYM
could maintain stable but lower crop yields.
Where combinations of NPK and FYM are given
the latter could contribute 20% to total
production.

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Mineral Fertilizer Use and the Environment
39
In Burkina Faso, fertilizers, manure and a
mixture were compared over 11 years on an
Oxisol. All treatments increased maize yields
during the first 3 to 4 years. Then during the
4-6 year period yields decreased for all
treatments. The mixture gave higher yields than
fertilizers alone over the 11 years. Fertilizer
increased acidity. When soil acidity and
exchangeable aluminium were neutralized by
liming, the yield increased. Manure helped to
limit the consequences but is incapable of
neutralizing all the acidity induced by the
fertilizer.
Speaking about the situation in India,
N.E. Borlaug (1996) pointed out that there was
not enough organic manure available in India to
supply sufficient nutrients to produce the food
grain needed to feed the population. The supply
of nutrients from organic manure was insufficient
to compensate for nutrient depletion and the
nutrient supply from this source was unlikely to
improve due to the competing demand for
alternative uses such as fuel, fodder and feed.
Comparing the situation in China with that of
India, he said that one of the reasons for the
higher use of organic matter in China was the fact
that the government subsidized coal, thus
reducing the need to use organic materials as
fuels.
In many African and Asian countries animal
wastes and crop residues have competing uses
and the problem is one of a shortage rather than
a surplus as in Europe and elsewhere. The
systems produce too little biomass, and much of
what is produced, is consumed by grazing
animals and then deposited elsewhere. The
return of organic matter to the soil is negligible.
Precautions to avoid the application of toxic
substances in organic amendments must
evidently be taken. Also the application of
manure under anaerobic conditions, for example
in rice paddies, should be avoided to prevent the
release of methane. Apart from this, the
integration of mineral and organic fertilization is
strongly recommended.